Slurries for chemical mechanical polishing

ABSTRACT

Novel slurries for the chemical mechanical polishing of thin films used in integrated circuit manufacturing. A tungsten slurry of the present invention comprises an oxidizing agent, such as potassium ferricyanide, an abrasive such as silica, and has a pH between two and four. The tungsten slurry of the present invention can be used in a chemical mechanical planarization process to polish back a blanket deposited tungsten film to form plugs or vias. The tungsten slurry can also be used to polish copper, tungsten silicide, and titanium nitride. A second slurry, which is a 9:1 dilution of the tungsten slurry is ideal for chemical mechanical polishing of titanium nitride films. A third slurry of the present invention comprises a fluoride salt, an abrasive such as silica and has a pH≦8. The third slurry can be used to polish titanium films.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to the field of semiconductor integratedcircuit manufacturing, and more specifically, to improved slurries forthe chemical mechanical polishing (CMP) of thin films used insemiconductor integrated circuit manufacturing.

2. Description of Relevant Art

Today, integrated circuits are made up of literally millions of activedevices formed in or on a silicon substrate or well. The active deviceswhich are initially isolated from one another are later connectedtogether to form functional circuits and components. The devices areinterconnected together through the use of well-known multilevelinterconnections. A cross-sectional illustration of a typical multilevelinterconnection structure 100 is shown in FIG. 1. Interconnectionstructures normally have a first layer of metallization, aninterconnection layer 102 (typically aluminum alloys with up to 3%copper), a second level of metallization 104, and sometimes a third oreven fourth level of metallization. Interlevel dielectrics 106 (ILDs),such as doped and undoped silicon dioxide (SiO₂), are used toelectrically isolate the different levels of metallization in siliconsubstrate or well 108. The electrical connections between differentinterconnection levels are made through the use of metallized vias 110formed in ILD 106. In a similar manner, metal contacts 112 are used toform electrical connections between interconnection levels and devicesformed in well 108. The metal vias 110 and contacts 112, hereinafterbeing collectively referred to as "vias" or "plugs", are generallyfilled with tungsten 114 and generally employ an adhesion layer 116 suchas TiN. Adhesion layer 116 acts as an adhesion layer for the tungstenmetal layer 114 which is known to adhere poorly to SiO₂. At the contactlevel, the adhesion layer acts as a diffusion barrier to prevent W andSi from reacting.

In one process which has presently gained wide interest, metallized viasor contacts are formed by a blanket tungsten deposition and a chemicalmechanical polish (CMP) process. In a typical process, via holes 202 areetched through an ILD 204 to interconnection lines or a semiconductorsubstrate 206 formed below. Next, a thin adhesion layer 208, such asTiN, is generally formed over ILD 204 and into via hole 202, as shown inFIG. 2b. Next, a conformal tungsten film 210 is blanket deposited overthe adhesion layer and into the via 202. The deposition is continueduntil the via hole 202 is completely filled with tungsten. Next, themetal films formed on the top surface of ILD 204 are removed by chemicalmechanical polishing, thereby forming metal vias or plugs 220.

In a typical chemical mechanical polishing process, as shown in FIG. 2c,the substrate or wafer 200 is placed face-down on a polishing pad 212which is fixedly attached to a rotatable table 214. In this way, thethin film to be polished (i.e., tungsten film 210) is placed in directcontact with pad 212. A carrier 216 is used to apply a downward pressureF₁ against the backside of substrate 200. During the polishing process,pad 212 and table 214 are rotated while a downward force is placed onsubstrate 200 by carrier 216. An abrasive and chemically reactivesolution, commonly referred to as "slurry" 222 is deposited onto pad 212during polishing. The slurry initiates the polishing process bychemically reacting with the film being polished. The polishing processis facilitated by the rotational movement of pad 212 relative to wafer200 as slurry is provided to the wafer/pad interface. Polishing iscontinued in this manner until all of the film on the insulator isremoved.

Slurry composition is an important factor in providing a manufacturablechemical mechanical polishing process. Several different tungstenslurries have been described in literature. One slurry described in"Chemical Mechanical Polishing for Fabricating Patterned W MetalFeatures as Chip Interconnects" [F. B. Kaufman, et al., Journal of theElectrochemical Society, Vol. 138, No. 11, November 1991], describes aslurry comprising potassium ferricyanide having a pH adjusted to greaterthan 5. It has been found that slurries with a pH greater than five formplugs 220 which are recessed below the insulating layer, as shown inFIG. 2d. Such recessing causes a non-planar via layer to be formed whichimpairs the ability to print high resolution lines during subsequentphotolithography steps and can cause the formation of voids or opencircuits in the metal interconnections formed above. Additionally, therecessing of plug 220 increases when overpolishing is used to ensurecomplete removal of the tungsten film across the surface of a wafer. Therecessing is further compounded when soft polishing pads are used duringpolishing (soft polishing pads are thought to help provide high anduniform polishing rates). As such, high pH slurries have been foundinadequate to manufacturably polish tungsten layers in an integratedcircuit.

On the other hand, slurries with low pH's (i.e., pH's<2) have been foundto provide high removal rates, good uniformity, and small recessing ofthe plugs. Unfortunately, however, slurries with pH's less than two areconsidered hazardous materials and therefore require special handlingprocedures which substantially increase manufacturing costs.Additionally, low pH slurries readily react and cause corrosion of thepolishing apparatus. As such, low pH slurries have been found inadequateto manufacturably chemically mechanically polish films in an integratedcircuit process.

As such, what is desired are slurries for chemical mechanical polishingof thin films used in integrated circuit manufacturing which do not formrecessed plugs and which are not hazardous or corrosive.

SUMMARY OF THE INVENTION

A novel slurry for chemical mechanical polishing (CMP) of films used inintegrated circuit manufacturing is described. The first slurry of thepresent invention comprises approximately 0.1 molar potassiumferricyanide, approximately 5% silica by weight, and has a pH adjustedto a value less than four and greater than two, with approximately3.4-3.6 being preferred. Concentrated acetic acid can be used to adjustthe pH of the slurry to the desired level. Additionally, the firstslurry can further comprise potassium acetate to help buffer the slurryand lubricate the polishing process. The first slurry can be used in aCMP process where a tungsten, tungsten silicide, copper, or titaniumnitride film in an integrated circuit is planarized, or polished backinto plugs or interconnections. A second slurry can be made by dilutingthe first slurry at a ratio of approximately 9:1 with deionized water(i.e., 9 parts deionized water to 1 part tungsten slurry). The secondslurry can be used for polishing titanium nitride films. A third slurrycomprising approximately 0.5 molar potassium fluoride, approximately0.5% silica by weight, and having a pH of approximately 5.2 can be usedfor polishing titanium films.

An object of the present invention is to provide slurries which can beused in a CMP process to form plugs or interconnects which are notrecessed below the ILD layer.

Another object of the present invention is to provide slurries which arenonhazardous and noncorrosive.

Yet another object of the present invention is to provide slurries whoseeffluent can be recycled and treated by conventional methods.

Still yet another object of the present invention is to provide slurrieswhich are manufacturably cost effective.

Still yet another objective of the present invention is to provideslurries which have a high and uniform polish rate across the surface ofa wafer and from wafer to wafer.

Still yet another objective of the present invention is to provideslurries which allow overpolishing in a CMP process withoutsignificantly recessing formed vias or interconnections.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional illustration showing a portion of a standardmultilevel integrated circuit.

FIG. 2a is a cross-sectional illustration showing the formation of a viahole through an insulating layer formed on a conductive layer of asemiconductor substrate.

FIG. 2b is a cross-sectional illustration showing the formation of anadhesion layer and a tungsten layer on the substrate of FIG. 2a.

FIG. 2c is a cross-sectional illustration of a chemical mechanicalpolishing apparatus used to polish the films formed on the substrate ofFIG. 2b.

FIG. 2d is a cross-sectional illustration showing the formation of aplug which has been recessed below the interlayer dielectric.

FIG. 3a is a cross-sectional illustration of a portion of a substrateshowing the formation of a via hole through an interlayer dielectricformed on a conductive layer of a semiconductor substrate.

FIG. 3b is a cross-sectional illustration showing the formation of anadhesion layers and a tungsten layer on the substrate of FIG. 3a.

FIG. 3c is a cross-sectional illustration showing a chemical mechanicalpolishing apparatus which can be used to chemical mechanical polish thefilms formed on the substrate of FIG. 3b.

FIG. 3d is a cross sectional illustration showing the formation of atungsten plug after chemical mechanically polishing the substrate ofFIG. 3b.

FIG. 3e is a cross-sectional illustration showing the formation of aninterconnect line over the planar plug and interlayer dielectric of FIG.3d.

FIG. 4a is a cross-sectional illustration showing the patterning of aninterlayer dielectric for the formation of interconnection lines.

FIG. 4b is a cross-sectional illustration showing the formation of abarrier layer and a copper layer on the substrate of FIG. 4a.

FIG. 4c is a cross-sectional illustration showing the formation ofinterconnections by chemical mechanical polishing of the substrate ofFIG. 4b.

DETAILED DESCRIPTION OF THE PRESENT INVENTION

Novel slurries for chemical mechanical polishing of thin films used inhigh density integrated circuits are described. In the followingdescription numerous specific details are set forth, such as specificmachinery materials, thicknesses, etc., in order to provide a thoroughunderstanding of the present invention. It will be obvious, however, toone skilled in the art that the present invention may be practicedwithout these specific details. In other instances, other well-knownsemiconductor processes and machinery have not been described inparticular detail in order to avoid unnecessarily obscuring the presentinvention.

The present invention describes novel slurries for the chemicalmechanical polishing (CMP) of thin films used in a semiconductorintegrated circuit. The novel slurries and CMP processes of the presentinvention are preferably used to form a via connection or plug betweenconductive layers of a semiconductor device. The teachings of thepresent invention, however, can be applied to other processes in themanufacture of semiconductor integrated circuits including, but notlimited to, the formation of interconnections and the planarization ofvarious layers. In fact, the teachings of the present invention can beapplied to CMP processes used in areas other than semiconductorprocessing.

In the fabrication of a contact or via connection between two conductivelayers of an integrated circuit, a semiconductor substrate or wafer 300is provided. Substrate 300 at this point has a conductive layer 301 asthe top most layer. The conductive layer 301 can be any one of a varietyof conductive materials used in semiconductor circuit manufacturingincluding but not limited to a metal layer, a semiconductor such assilicon, a doped semiconductor, a polysilicon layer, or a metal silicidelayer. An interlayer dielectric (ILD) 302 is formed over conductivelayer 301. Interlayer dielectric 302 is a thin film insulator which isgenerally an undoped silicon dioxide (SiO₂) formed by plasma enhancedCVD of TEOS between interconnection layers. A phosphosilicate (PSG) orborophosphosilicate (BPSG) film is generally used between polysiliconand metal layers. It is to be appreciated that other insulating layers,such as silicon nitride, or multilayer composite dielectrics, includingsuch things as spin on glass, may also be used. The function ofinterlayer dielectric 302 is to electrically isolate conductive layer301 from a subsequently formed conductive layer. Interlayer dielectric302 can be formed by techniques well-known in the art.

First, an opening or via hole 304 is formed in interlayer dielectric302. A photoresist layer is formed over ILD 302 which is then masked,exposed, and developed with techniques well-known in the art to definethe location for via hole 304. The insulating layer 302 is thenanisotropically etched with techniques well-known in the art to form viahole 304. Via hole 304 is etched until conductive layer 301 is reached.A via hole with substantially vertical side walls and a large aspectratio (aspect ratio=via depth/via width) is desired. Such a via hole iscompatible with the high packing density required for future ultra largescale integrated (ULSI) circuits.

Next, as shown in FIG. 3b, an adhesion layer or adhesions layers (ifused) are blanket deposited over ILD 302. In the preferred method of thepresent invention, an approximately 200 Å titanium (Ti) contact layer305 is blanket deposited over the top surface of ILD 302, on the sidesof ILD 302 in via hole 304 and on conductive layer 301 in via hole 304.The function of titanium contact layer 305 is to decrease the contactresistance of the fabricated plug in order to improve electricalperformance. Titanium layer 305 also acts as a polish stop for tungstenand/or TiN polish steps described below. Titanium contact layer 305 isformed to a thickness of approximately 200 Å and can be formed bywell-known means, such as sputtering from a titanium target. Next, atitanium nitride (TIN) layer 306, of a thickness of approximately 600 Å,is blanket deposited over titanium layer 305. Titanium nitride layer 306can be formed by any one of a plurality of well-known techniques,including but not limited to, reactive sputtering from a titanium targetin a nitrogen atmosphere and chemical vapor deposition (CVD). Titaniumnitride layer 306 provides an adhesion layer and a diffusion barrier fora subsequently deposited tungsten layer which is known to have pooradhesion to insulators like SiO₂, and high reactivity with metals suchas aluminum and titanium. Efforts should be made to form titanium layer305 and titanium nitride layer 306 as conformally as possible so thathigh aspect ratio vias can be reliably formed. It is to be appreciatedthat other adhesion layers, such as tungsten silicide formed by chemicalvapor deposition, may be used if desired.

Next, as also shown in FIG. 3b, a tungsten (W) layer 308 is blanketdeposited over TiN layer 306. The deposition completely fills via hole304 with tungsten. The deposition forms a thick tungsten layer on thetop surface of ILD 302. A slight dimple 309 may result in the topportion of tungsten layer 308 over via hole 304. Tungsten layer 308 isformed to a thickness between 4000-5000 Å for an 4500 Å diameter plug.Tungsten layer 308 can be formed by CVD using hydrogen (H₂) reduction oftungsten hexafluoride (WF₆) wherein the ratio of H₂ to WF₆ isapproximately 7:1. An Applied Materials Precision 5000 deposition systemcan be used for depositing tungsten layer 308. The tungsten layeradheres well to underlying TiN adhesion layer 306 and the titanium layer305 forms a good ohmic contact with conductor layer 301 below.

Next, as shown in FIG. 3c, tungsten layer 308, titanium nitride layer306, and titanium layer 305 are chemical mechanically polished back toform metallized plugs. In a typical CMP process, substrate 300 is placedface down on a polishing pad 310 attached to a rotatable table 312. Inthis way the thin film to be polished (i.e. tungsten film 308) is placedin direct contact with polishing pad 310. A carrier 316 is used toforcibly press substrate 300 down against polishing pad 310 duringpolishing. A slurry is deposited onto polishing pad 310 from a nozzle320 during polishing. Slurry chemically passivates or oxidizes the thinfilm being polished and then abrasively removes or polishes off thepassivated surface. The removal of the thin film is facilitated by thechemically reactive slurry as pad 310 and substrate 300 are rotatedrelative to one another under a polishing pressure F₁ applied by carrier316. Polishing is continued in this manner until the desiredplanarization is achieved or the desired amount of film is removed.

Polishing pad 310 can be formed of a variety of different materials. Forexample, polishing pad 310 can be a hard pad such as the IC-60 padmanufactured by Rodel Corporation. Additionally, polishing pad 310 canbe a relatively soft pad such as the Polytech Supreme pad alsomanufactured by Rodel Corp. A soft polishing pad is thought to provideimproved polish removal rates and improved uniformity. What isimportant, however, is for polishing pad 310 to adequately and uniformlydeliver slurry across the entire wafer/pad interface. A plurality ofperformed grooves can be added to pad 310 to help transport slurry aboutthe wafer/pad interface. Additionally, slurry need not be simplydeposited onto pad 310 from a nozzle 320, as shown in FIG. 3c, butrather can be pumped through the polishing pad directly to the wafer padinterface as described in copending U.S. patent application, Ser. No.08/103,412 filed Aug. 6, 1993 and assigned to the present assignee. Sucha technique allows for a fast and manufacturable transition betweendifferent slurry types. Additionally, polishing pad 310 need notnecessarily rotate to facilitate abrasive polishing, but rather may movein other directions, such as in an orbital direction with a radius lessthan the substrate radius, as described in U.S. patent application Ser.No. 08/103,412 filed Aug. 6, 1993 and assigned to the present assignee.

A carrier similar to carrier 316 can be used to forcibly press androtate wafer 300 against polishing pad 310 during polishing. A shaft 322is used to apply a downward force F₁ (between 2-12 psi) and to rotatesubstrate 300 during polishing. An ordinary retaining ring 324 can beused to prevent substrate 300 from slipping laterally during polishing.An insert pad 326 is preferably used to cushion substrate 300 fromcarrier 316. Wet surface tension or vacuum pressure can be used to holdwafer 300 in place. It is to be appreciated, however, that othercarriers such as curved carriers or the improved carriers described inco-pending U.S. patent application Ser. No. 08/103,918 filed Aug. 6,1993 and assigned to the present assignee can be used without departingfrom the scope of the present invention.

The key to obtaining good and manufacturable CMP results in the presentinvention are the novel slurries utilized in the chemical mechanicalpolishing (CMP) process. A novel tungsten slurry is used to polish backtungsten film 308. The novel tungsten slurry of the present inventioncomprises an oxidizing agent capable of oxidizing tungsten, an abrasivefor physically removing ;the oxidized tungsten, and has a pH between twoand four. The pH is low enough to prevent plug recessing but yet is highenough so that the slurry is nonhazardous. A slurry having a pH between3.4 to 3.6 is preferred in the present invention because it provides agood balance between plug recessing and handling hazards and costs.

Oxidizing agents including, but not limited to, potassium ferricyanide,potassium dichromate, potassium iodate, potassium bromate, and vanadiumtrioxide, can be used in the tungsten slurry. Potassium ferricyanide (K₃Fe(CN)₆) is the preferred oxidizing agent in the present inventionbecause it does not readily precipitate out of solution, causecorrosion, nor is it classified as a hazardous material. Additionally,potassium ferricyanide has been found to be a good oxidant for materialssuch as tungsten, tungsten silicide, copper, and titanium nitride films,all used in integrated circuit manufacturing. The slurry has aconcentration of oxidant high enough to sufficiently oxidize the entiresurface area of the film being polished, yet low enough so that it canbe dissolved in the slurry (i.e., a concentration less than its solidsolubility). A slurry comprising between 0.01 to 0.3 molar potassiumferricyanide has been found to provide sufficient results. A slurrycomprising approximately 0.1 molar potassium ferricyanide (i.e. 32.9grams per liter of potassium ferricyanide) is preferred because itprovides a sufficient amount of oxidizing agent (approximately ten timesmore than stoichiometrically necessary), and yet is a concentrationsmall enough to keep slurry costs down. It is also to be appreciatedthat the tungsten slurry of the present invention can contain up toapproximately 3.0 molar potassium ferricyanide (i.e., the solidsolubility of potassium ferricyanide) if desired. Such a large amount ofpotassium ferricyanide, however, will significantly increase slurrycosts and will increase the polish rate to an uncontrollable level.

An abrasive, such as silica, alumina, or ceria, is provided in theslurry to physically or mechanically strip the passivated surface of themetal being polished. Silica is the preferred abrasive in the presentinvention because it can be used without scratching the surface of thematerial being polished. The tungsten slurry of the present inventionutilizes a colloidal silica comprising between 1-25% by weight silica,with approximately 5% by weight being preferred. This amount of silicaprovides an optimum balance between polish removal rate and goodselectivity to interlayer dielectrics used in integrated circuitmanufacturing. Additionally, this amount of abrasive is sufficient toabrasively "buff" or polish the film, but yet is low enough to keepslurry costs down. The preferred colloidal silica is manufactured byCabot, Inc. and sold under the tradename Semi-Sperse™-25.(Semi-Sperse™-25 comprises approximately 25% by weight silica, a smallamount of KOH and deionized water.) An alternative colloidal Silica isCab-O-Sperse® also manufactured by Cabot, Inc. (Cab-O-Sperse® comprisesapproximately 15% weight percent silica and the remainder deionizedwater.)

It is recommended to provide a small amount (between 0.05 to 0.005molar) potassium acetate (CH₃ COOK) in the tungsten slurry. Potassiumacetate buffers the chemistry (i.e., stabilizes the pH). Additionally,it is suspected that potassium acetate helps to lubricate the polish,thereby making the CMP process of the present invention moremanufacturable. Potassium acetate can be added directly to the slurry ormay be introduced through a reaction of slurry components. For example,potassium acetate can be formed by reaction of potassium from eitherpotassium ferricyanide or potassium hydroxide in the colloidal silica,and acetate from acetic acid used to adjust the pH of the slurry.

The preferred slurry composition for chemical mechanical polishing oftungsten films is a solution comprising approximately 0.1 molarpotassium ferricyanide, approximately 5% by weight silica, a smallamount of potassium acetate with the remainder deionized water. A smallamount of concentrated acetic acid is included to adjust the pH of thetungsten slurry to the desired range of 3.4 to 3.6. The tungsten slurryof the present invention can be formed by diluting Semi-Sperse™-25colloidal silica with deionized water until it is approximately 10%silica by weight. The diluted Semi-Sperse™-25 colloidal silica is thenmixed at a 1:1 ratio with the approximately 0.2 molar potassiumferricyanide. A small amount of acetic acid can then be added to theslurry to adjust the pH to the preferred range of 3.4 to 3.6.

The preferred composition of the novel tungsten slurry of the presentinvention exhibits many properties and qualities which make the chemicalmechanical polishing process of the present invention extremelymanufacturable. The slurry exhibits a high tungsten polish removal rateof approximately 1600-2400 Å/min, providing good wafer throughput.Additionally, the polish removal rate is very uniform with less than 10%deviation over three sigma. (It will be understood by those skilled inthe art that such things as pad types, polish pressure, rotation rates,etc., effect polish removal rate and polish uniformity.) Additionally,the slurry is much more selective to tungsten than to insulatorstypically used in semiconductor manufacturing. For example, the slurryhas a tungsten/SiO₂ selectivity of greater than 25:1 and a tungsten BPSGselectivity of greater than 4:1. In this way overpolishing can be usedwithout significantly attacking insulating layers formed below andcausing potential reliability problems. Still further, the tungstenslurry of the present invention can retain its consistency indefinitely,allowing the tungsten slurry to be premixed. It is to be appreciatedthat prior art slurries tend to degrade and precipitate out activeingredients over time. Additionally, the tungsten slurry chemistry ofthe present invention allows slurry effluent (waste) to be recycled onceor more without treatment. This provides a substantial savings in bothslurry consumption costs and effluent disposal costs. Still further,since the slurry of the present invention utilizes noncorrosivepotassium ferricyanide as the active ingredient, a clean and anenvironmentally safe CMP process is provided. Additionally, the wastefrom the tungsten CMP process can be treated by presently availablewaste treatment processes. The tungsten slurry of the present inventioncan also be used to polish other materials used in integrated circuitmanufacturing, including but not limited to tungsten silicide, copperand titanium nitride.

The abrasive polishing of tungsten layer 308 is continued in theabove-described manner until substantially all of the tungsten layer 308formed on the titanium nitride layer 305 formed over the top surface ofthe interlayer dielectric 302 is removed. Next, TiN adhesion layer 306(if used) is chemically mechanically polished. The TiN adhesion layercan be polished with the tungsten slurry described above. The polishremoval rate of TiN with the tungsten slurry, however, is low (˜450Å/min). A low TiN polish rate coupled with a high tungsten polish ratecan result in significant plug recessing. It is, therefore, preferred todilute the tungsten slurry with deionized water at a ratio of 9:1 (i.e.9 parts deionized water to 1 part tungsten slurry) to form a titaniumnitride slurry. By diluting the tungsten slurry the titanium nitrideslurry comprises approximately 0.01 molar potassium ferricyanideapproximately 0.5% silica by weight and has a pH between 2 and 4. Bydiluting the tungsten slurry, the TiN polish removal rate increases toapproximately 800 Å/min while the tungsten polish rate decreases to lessthan <1000 Å/min. In this way the TiN adhesion layer is removed at ahigh enough rate to allow for good wafer throughput. More importantly,however, by decreasing the tungsten polish rate to approximately thesame rate as the TiN polish rate, the tungsten plug is not"overpolished" or recessed while polishing the TiN adhesion layer. TiNadhesion layer 306 is polished until all of the TiN formed on titaniumlayer 305 over the top surface of ILD 302 is removed. Since the tungstenslurry (standard or diluted) does not significantly polish titanium,titanium layer 305 effectively acts as a polish stop for the TiN polishstep. This helps to make the CMP process of the present invention verymanufacturable.

Next, titanium layer 305 (if used) formed on the top surface of ILD 302is removed by chemical mechanical polish. In the preferred embodiment ofthe present invention, a novel titanium slurry is used to facilitate thechemical mechanical polishing of titanium layer 305. The novel titaniumslurry of the present invention comprises a fluoride salt, an abrasive,and has a pH≦8. The fluoride salt acts as a complexing agent to complexthe titanium film, and the abrasive mechanically strips the complexedtitanium surface. The complexing agent preferably is a fluoride saltincluding, but not limited to, sodium fluoride and potassium fluoride.The slurry should have a fluoride salt concentration high enough tosufficiently complex the entire titanium surface, but yet low enough tokeep slurry costs down. The preferred composition of the titanium slurrycomprises approximately 0.5 molar potassium fluoride. The abrasive canbe any one of a variety of well-known abrasives including, but notlimited to, silica, ceria and alumina. The preferred composition of thetitanium slurry of the present invention comprises approximately 0.5%silica by weight which provides the slurry with good selectivity totitanium over oxides. In order to increase the polish rate of thetitanium slurry the amount of silica in the slurry can be increased toas high as 15% by weight. It is to be appreciated, however, that such alarge amount of silica will reduce the titanium/oxide selectivity of theslurry.

The novel titanium slurry of the present invention has a pH≦8, andpreferably a pH adjusted to approximately 5.2. The pH of the titaniumslurry can be adjusted by adding acetic acid or other well-known acids.The pH of the titanium slurry is known to effect polish removal rates.The titanium slurry exhibits a titanium polish removal rate ofapproximately 250 Å/min when the pH is approximately 5.2. Additionally,with a pH of 5.2, the titanium slurry polishes titanium nitride at arate of approximately 40 Å/min and the polish rate of tungsten isundetectable. In this way, titanium layer 305 can be polished withoutrecessing or overpolishing the tungsten plug. Additionally, with thenovel titanium slurry, titanium nitride adhesion layer 306 inside thevia is not significantly removed during titanium polishing and thereforeno "etch-our" of the plug will result. Still further, the titaniumslurry is more selective to titanium than to ILDs such as SiO₂ and BPSG.This allows overpolishing to be utilized. It is to be noted that if thetitanium slurry and the tungsten slurry are mixed, significant plugrecessing may result. As such, it is imperative that one thoroughlyrinse the wafer and polishing pad with deionized water prior topolishing titanium film 305. It is further advised to use a separatemachine for the Ti polish in order to ensure no residual tungsten slurryis present during the titanium polishing.

At the completion of the polishing process, a filled via hole ortungsten plug 322 is formed. Tungsten plug 322 is substantially planarwith the top surface of ILD 302. It is to be stressed that in thepresent invention tungsten plug 322 is not recessed into ILD 302. Infact, there is very little recessing even when a soft polishing pad isutilized. More importantly, tungsten plug 322 exhibits onlyapproximately 500 Å of recessing when overpolishing by 25%.Overpolishing guarantees that all conductors formed on ILD 302 arecompletely removed so that no electrical shorts may result. The abilityto overpolish without significant recessing of plug 322 makes thepresent process extremely manufacturable.

After completion of the CMP process, as shown in FIG. 3e, aninterconnect line 324 is formed on ILD 302 and on tungsten plug 322.Interconnect line 324 can be formed by blanket depositing a conductivelayer of, for example, aluminum alloys, tungsten, copper, etc., over ILD302 and tungsten plug 322. The conductive layer is then covered by aphotoresist layer which is then masked, exposed, and developed withtechniques well-known in the art, to define the location of interconnect324. It is to be appreciated that very narrow, high resolution lines canbe printed because of the extremely planar plug previously formed by thepresent invention. The conductive layer is then etched by techniqueswell-known in the art to form interconnect line 324. It is to beappreciated that interconnect line 324 is substantially planar due tothe planar plug formed below. In this way the potential for opencircuits which can develop in interconnection lines formed over largestep heights, such as recessed plugs, are eliminated. The fabrication ofa planar, highly reliable, low resistance, high density electricalconnection between two conductive layers of an integrated circuit is nowcomplete.

It is to be appreciated that the novel slurries and chemical mechanicalpolishing processes of the present invention can be used to fill-ingrooves other than contacts or vias and can be used to polish metalsother than tungsten, tungsten silicide, TiN, and titanium. For example,the tungsten slurry and CMP process of the present invention can beapplied to the formation of a copper interconnection layer. In thisembodiment, as shown in FIG. 4a, an insulating layer 402 is patterned toprovide openings or grooves where interconnection lines are to beformed. An adhesion layer/diffusion barrier, such as TiN, is then formedover the insulating layer and into the grooves covering the sides andthe bottom of the groove, as shown in FIG. 4b. A copper layer is thenformed with well-known techniques over the adhesion layer and depositeduntil the grooves are substantially filled. The copper layer and theadhesion layer on the top surface of the insulating layer are thenchemically mechanically polished back, as in the plug process, as shownin FIG. 4c. The novel tungsten slurry of the present invention can beused to polish copper. The tungsten slurry has a good copper polish rateof approximately 2000 Å/min and provides good local and global polishuniformity. It is to be appreciated that copper is patterned poorly withstandard dry etching techniques. The slurries and chemical mechanicalprocess of the present invention make possible the use of highperformance planar, copper interconnection lines.

It is to be appreciated that the slurries of the present invention havebeen described in particular detail with respect to preferred processesand structures for forming planar plugs and interconnections. Thepreferred composition of the slurries are ideally suited for thedescribed processes. The present invention, however, is not intended tobe limited to these preferred slurry compositions, via structures and/orCMP processes. One skilled in the art will readily recognize that theactual composition of the slurries can and probably should be optimizeddepending upon the specific metals, insulators and processes actuallyused. The scope of the present invention is intended to be defined bythe claims which follow.

Thus, novel slurries and CMP processes for chemical mechanical polishingof thin films used in semiconductor integrated circuit manufacturinghave been described.

We claim;
 1. A slurry for chemically mechanically polishing a thin filmused in an integrated circuit comprising:an oxidizing agent for saidfilm; an abrasive; and water;wherein said slurry has a pH greater thantwo and less than four.
 2. The slurry of claim 1 wherein said oxidizingagent is potassium ferricyanide.
 3. The slurry of claim 2 wherein saidslurry comprises between 0.01-0.3 molar potassium ferricyanide.
 4. Theslurry of claim 2 wherein said slurry comprises approximately 0.1 molarpotassium ferricyanide.
 5. The slurry of claim 1 comprising 1-25% silicaby weight.
 6. The slurry of claim 5 wherein said slurry comprisesapproximately 5% silica by weight.
 7. The slurry of claim 1 wherein saidpH of said slurry is between 3.4 to 3.6.
 8. The slurry of claim 1further comprising potassium acetate.
 9. A slurry for chemicallymechanically polishing a film selected from the group consisting oftungsten and titanium nitride;potassium ferricyanide; silica; andwater;wherein said slurry has a pH greater than two and less than four.10. The slurry of claim 9 wherein said film is tungsten and said slurrycomprises between 0.01-0.3 molar potassium ferricyanide.
 11. The slurryof claim 9 wherein said film is tungsten and said slurry comprisesapproximately 0.1 molar potassium ferricyanide.
 12. The slurry of claim9 wherein said film is tungsten and said slurry comprises approximately5% silica by weight.
 13. The slurry of claim 9 wherein said film istungsten and said slurry further comprises acetic acid.
 14. The slurryof claim 9 wherein said film is tungsten and said slurry furthercomprises potassium acetate.
 15. The slurry of claim 9 wherein said filmis tungsten and said slurry comprises approximately 0.1 molar potassiumferricyanide;approximately 5% silica by weight; and water;wherein saidslurry has a pH of greater than two and less than four.
 16. The slurryof claim 9 wherein said films titanium nitride and said slurry comprisesapproximately 0.01 molar potassium ferricyanide.
 17. The slurry of claim9 wherein said film is titanium nitride and said slurry comprisesapproximately 0.5% silica by weight.
 18. The slurry of claim 9 whereinsaid film is titanium nitride and said slurry comprises approximately0.01 molar potassium ferricyanide and approximately 0.5% silica byweight.
 19. The slurry of claim 9 wherein the film is titanium nitrideand said slurry further comprises acetic acid.
 20. The slurry of claim 9wherein the film is titanium nitride and said slurry further comprisespotassium acetate.